1. Field of the Invention
This invention relates to the field of alternating phase-shifting masks, and in particular to a method of correcting three-dimensional (3D) effects in alternating phase-shifting masks using sub-resolution features.
2. Description of Related Art
To fabricate an integrated circuit (IC), a physical representation of the features of the IC, e.g. a layout, is transferred onto a plurality of masks. Note that as used herein, the term “mask” includes “reticles”. The features make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on. A mask is generally created for each layer of the IC. To create a mask, the data representing the layout for a corresponding IC layer can be input into a device, such as an electron beam machine, which writes IC features onto the mask. Once a mask has been created, the pattern on the mask can be transferred onto the wafer surface using a lithographic process.
Lithography is a process whose input is a mask and whose output includes the printed patterns on a wafer. As printed patterns on the IC become more complex, a need arises to decrease the feature size. However, as feature sizes shrink, the resolution limits of current optical-based lithographic systems are approached. Specifically, one type of lithographic mask includes clear regions and opaque regions, wherein the pattern of these two regions defines the features of a particular semiconductor layer. Under exposure conditions, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the lithographic process.
Various techniques have been proposed to improve this resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves. In this manner, the projected images from these two regions destructively interfere where their edges overlap, thereby improving feature delineation and allowing greater feature density on the IC. A mask that uses such techniques is called a phase shifting mask (PSM).
In one type of PSM, called an alternating PSM, apertures between closely spaced features are processed so that light passing through any aperture is 180 degrees out of phase from the light passing through an adjacent aperture. FIGS. 1A and 1B illustrate one embodiment of an alternating PSM 100 including closely spaced opaque (e.g. chrome or some other absorbing material) features 101, 102, 103, and 104 formed on a transparent, e.g. quartz, substrate 105. Thus, apertures 106, 107, and 108 are formed between features 101-104.
To provide the phase shifting in this embodiment, the areas of substrate 105 under alternating apertures can be etched, thereby causing the desired 180 degree phase shift. For example, substrate 105 can be etched in the area defined by aperture 107 to a predetermined depth. In this manner, the phase shift of light passing through aperture 107 relative to light passing through apertures 106 and 108 is approximately 180 degrees.
Unfortunately, the use of an alternating PSM can introduce an intensity imbalance problem. FIG. 1C illustrates a graph 130 that plots intensity (0 to 1.0) versus position on alternating PSM 100. In graph 130, waveforms 131 that are shown nearing 1.0 intensity correspond to apertures 106 and 108, whereas waveform 132 that is shown at approximately 0.84 intensity corresponds to aperture 107. The intensity imbalance between the 180 degree phase shifting region (i.e. aperture 107) and the 0 degree phase shifting regions (i.e. apertures 106 and 108) is caused by the trench cut into substrate 105, thereby causing diffraction in the corners of aperture 107 and degrading the intensity of the corresponding waveform. This industry-recognized diffraction effect is called a three-dimensional (3D) effect.
Intensity imbalance can adversely affect printing features and overlay on the wafer. Typically, a feature on a binary mask has a pair of corresponding phase shifting regions on an alternating PSM. For example, referring to FIG. 1D, a feature 140 can have a corresponding 0 degree phase shifting region (also called a phase shifter herein) 141 placed relative to one side of feature 140 and a corresponding 180 degree phase shifter 142 placed relative to the other side of feature 140. Of interest, if phase shifters 141 and 142 are the same size, the electric field associated with phase shifter 141 is stronger than the electric field associated with phase shifter 142, thereby resulting in the maximum interference of these fields to occur to the right of centerline 143 on feature 140. Thus, under these conditions, feature 140 will actually print on the wafer to the right of the desired location as shown by dotted lines indicating the printed location of feature 150 and its associated centerline 153.
Moreover, any defocus in the system can exacerbate the 3D effect and cause significant deviation from desired feature placement on the wafer. Because any wafer production line requires at least some acceptable range of defocus, e.g. typically within 0.4 microns, feature placement is frequently adversely affected when using an alternating PSM. Therefore, those in the industry have proposed various methods to address the intensity imbalance problem.
In one proposed method shown in FIG. 1E, an additional etching step can be performed on substrate 105, thereby providing an undercut etch 160 of features 101-104. Undercut etch 160 increases the intensity by attempting to localize the diffraction effects under features 101-104. Unfortunately, under-cut etch 160 can also create mechanical instability of features 101-104 on the mask. Specifically, the greater the under-cut etch, the greater the probability of mechanical (e.g. chrome) instability during subsequent processing steps, such as mask cleaning. Thus, undercut etch 160 provides an incomplete solution with the potential of causing complete mask failure.
Another potential solution (not shown) includes biasing the size of the 180 degree phase-shifting region to be larger than the 0 degree phase-shifting region, as described in U.S. Pat. No. 6,670,082, which issued on Dec. 30, 2003 to the assignee of the present application, Numerical Technologies, Inc. This method ensures mechanical stability, but does typically require determining the appropriate bias for a plurality of 180 degree phase-shifting regions.
Another potential solution (not shown) includes providing a dual trench structure in the PSM, i.e. both the 0 degree and 180 degree phase shifters are formed using trenches. Using this structure, both phase shifters suffer from diffraction effects, which can minimize or even eliminate the undesirable intensity imbalance. Unfortunately, forming the additional trenches in the substrate adds significantly more time to the manufacturing operation, e.g. typically doubling the time, thereby undesirably increasing manufacturing cost.
Yet another potential solution (not shown) includes sloping the sidewalls of the 180 degree phase shifters and providing a layer of chrome on such sloped sidewalls, thereby minimizing diffraction effects. However, this type of mask, called sidewall chrome alternating aperture mask (SCAAM), also suffers from significantly higher manufacturing cost than the standard PSM. Moreover, as features get smaller, the appropriate angle for the sloping sidewalls can become a limiting factor. That is, the angle may result in a de facto minimum feature size.
Therefore, a need arises for a method of correcting 3D effects of phase-shifting masks while ensuring mechanical stability and manufacturing efficiency.